The present invention methods and materials relate to the simple, efficient production of isotopically labeled formaldehyde for use in the formation of radiotracer compounds.
Synthesis of formaldehyde containing isotopes of carbon (11C, 13C and 14C, hereinafter denoted as *C) and isotopes of hydrogen (deuterium (D) and tritium (T)) are contemplated embodiments of this invention. A preferable embodiment of the present invention is the simple, efficient production of [11C]formaldehyde (11CH2O) for use in the formation of 11C-labeled radiotracer compounds for positron emission tomography.
In some embodiments, the hydrogen atoms of 11CH2O may be replaced with deuterium (11CD2O) or tritium (11CT2O). Deuterium-substituted radiotracers can have different pharmacokinetics, which can be used to alter the utility of various radiotracer compounds (Fowler, et al. (1994) J. Nucl. Med. 36:1255). The radioactive tritium substitution of various radiotracers can be useful for validation of in vivo imaging through in vitro radiographic studies as well as for validation of serum concentrations of the radiotracer compound through scintillation counting.
Carbon-13 and carbon-14 formaldehyde can be synthesized through the methods of the invention and used for various applications. For example, 13CH2O may be used in place of 11CH2O when developing a new synthetic route for the preparation of a new radiotracer compound. 14CH2O can be used to incorporate radioactivity into the same radiotracer compound which can then be used to validate in vivo imaging results through the use of the carbon-14 labeled compound for in vitro autoradiography studies and through scintillation counting.
Positron emission tomography (PET) is an analytical imaging technology which utilizes compounds labeled with positron emitting radioisotopes as molecular probes to image and measure biological and biochemical processes. To image biological processes using PET, atoms in particular biological compounds are replaced or substituted with the positron emitting radioisotopic atoms to form various radiotracer compounds. Oxygen, nitrogen and carbon atoms of organic compounds can be substituted with their positron emitting isotopes (15O, 13N and 11C). Because there are no positron emitting isotopes of hydrogen, the positron emitting fluorine-18 (18F) isotope is used as a substitute for hydrogen. Other, less frequently used positron emitting isotopes include those of Cu, Zn, K, Br, Rb, I, P, Fe, Ga and others. For the most typically used PET isotopes (O, N, C and F) the short half-life of the radioisotope demands that the synthetic chemical reaction incorporating the radioisotope be quick, efficient and of high yield, with little or no isotope dilution. In particular, for 11C, which has a half-life of 20.4 minutes, production of the radio-carbon through completion of the imaging scan must be accomplished in two to three hours (Fowler, et al. (1997) Acc. Chem. Res. 30:181-188). A typical preparation includes about 10 minutes for isotope production (generally in the form of 11CO2 or 11CH4), 40 to 60 minutes or less for radiotracer synthesis and up to about 90 minutes for PET imaging.
The incorporation of carbon-11 into small molecules has been paramount to the success of positron emission tomography for in vivo molecular imaging and drug research and development. However, many of the properties that make [11C] an ideal radionuclide for PET have impeded its chemical development. For instance, as noted above, the short half-life (t1/2=20.4 min) necessitates rapid chemical syntheses and purifications. Moreover, high specific activity, which makes it possible to image low concentration receptors and molecular targets, places the working concentration of [11C] labeling reagents in the low nanomolar range. But perhaps the biggest challenge in the synthesis of [11C]-labeled compounds is the lack of available labeling reagents.
Nearly all carbon-11 syntheses begin with a nuclear reaction [14N(p, α)11C] using a cyclotron or other accelerator to produce 11CO2 or 11CH4 from which labeling reagents are prepared.
By far the most common, almost canonical method, to label a molecule with [11C] is through methylation, typically with 11CH3I, which is simply prepared using commercially available reagents and equipment (e.g., General Electric TRACERlab™ FX C Pro). While methyl groups appear quite frequently in relevant compounds and [11C]-methylation has led to many successful radiotracers, reliance on methylation limits the range of potential radiotracer probe compounds. Consequently, there exists a need for new reaction development to focus on methods to incorporate [11C] in skeletal positions of target molecules. Several research groups have developed or adapted synthetic methods for 11C incorporation into benzene rings, carbocycles, and heterocycles as well as non-pendant locations. By using carefully designed synthetic organic reactions, each of these has expanded the types of radiotracers that can be accessed.
[11C]Formaldehyde has shown great promise as a labeling reagent for the preparation of PET compounds. Due to its versatile oxidation state, [11C]formaldehyde (11CH2O) provides a way to insert carbon-11 into compounds through routes that cannot be synthesized using the more readily available 11CH3I. For example, it has been used in synthesis of a variety of compounds through reductive methylations (Straatmann et al. (1975) J. Nucl. Med. 16:425; Marazano, et al. (1977) Int. J. App. Radiat. Isot. 28:49; Maziere, et al. (1977) J. Label. Comp. Radiopharm. (1977) 28:196; Berger et al. (1979) Int. J. App. Radiat. Isot. 30:393), ring-closure reactions (Nader et al. (1998) Appl. Radiat. Isot. 49:1599; Roeda, et al. (2002) J. Label. Comp. Radiopharm. 45:37; Van der Mey, et al. (2006) Bioorg. Med. Chem. 14:4526) and electrophilic aromatic substitutions (Langer, et al. (2005) J. Label. Comp. Radiopharm. 48:577) among others (e.g., Pike et al. (1984) Int. J. Appl. Radiat. Isot. 35:103).
However, the widespread development and use of synthetic methods employing [11C]formaldehyde in the preparation of PET-compounds has been hindered by its lack of availability to most radiochemistry facilities. Several methods have been developed for the synthesis of [11C]formaldehyde from [11C]methanol beginning in 1972 (Christman, et al. (1972) Proc. Natl. Acad. Sci. USA 69:988), improved over time with new catalysts (Roeda, et al. (2003) J. Label. Comp. Radiopharm. 46:449), and quite elegantly synthesized enzymatically (Slegers, et al. (1984) J. Nuc. Med. 25:338; Svärd, et al. (1984) J. Label. Comp. Radiopharm. 21:1175; Hughes, et al. (1995) Nucl. Med. Biol. 22:105). Although these methods have been developed for synthesis of [11C]formaldehyde from [11C]methanol, none have been adapted to be operable in the equipment available to the majority of radiochemists.
While each of these methods has found utility, they each have disadvantages preventing more widespread use. The previous methods for the preparation of [11C]formaldehyde have relied on the partial or the complete reduction of 11CO2 to 11CH3OH followed by oxidation, as shown in reaction schemes 1.1 and 1.2.
Partial Reduction:11CO2→11CH2O+H11COOH+11CH3OH  1.1Complete Reduction:11CO2→11CH3OH →11CH2O+H11COOH  1.2
Because these methods rely on a reduction step that occurs in solution, typically with lithium aluminum hydride, a reduction in specific activity occurs.
We sought to avoid these previously used routes and to make use of the widely available and gas-phase produced 11CH3I as the starting material for the efficient, simple production of 11C-formaldehyde. In developing the methods and materials we sought to bear in mind that for generalized utility it would be most useful if the production could be made without the need for new equipment, further bearing in mind that the reaction conditions needed to be mild and of short duration.